persist user sessions across reboots

By mounting a persistent volume at /var/lib/php/sessions.

To size up the volume, I looked at the current size of the directory:

root@patchdemo-75985f4cbc-sp7hz:/srv/app# du -sh /var/lib/php/sessions
5.4M	/var/lib/php/sessions

This was after one week of uninterrupted Patchdemo operation after the most recent deployment to production. I gave the volume roughly 20 times that size: 100M

That size should be plenty even accounting for potential future increase in number of users, given that PHP sessions are configured to expire after 2 weeks: https://gitlab.wikimedia.org/repos/qte/catalyst/patchdemo/-/blob/9574e8a8/setup.sh#L58

Bug: T374284